Silica glass optical fiber has a problem in that the transmission loss gradually increases as a result of long-term contact with hydrogen gas or water. The reason therefor is considered to be an absorption loss caused by molecular movement itself of hydrogen molecules diffused in optical fiber, an increase of absorption loss by hydroxy groups produced by the reaction of hydrogen with B.sub.2 O.sub.3, P.sub.2 O.sub.5, GeO.sub.2, Na.sub.2 O, etc. contained in optical fiber as dopants, and the like.
In view of the above, there have been so far proposed imparting resistance to water and hydrogen, to optical fiber by forming a carbon film on the surface of optical fiber upon thermal decomposition of hydrocarbon and chlorine-containing hydrocarbon. Nevertheless, a carbon film formed by conventional methods such as a method comprising introduction of a low temperature optical fiber into a high temperature reactor furnace, heating same to a necessary temperature, and thermal decomposition of carbon coat-forming gas; and a method comprising thermal decomposition of carbon coat-forming gas of hydrocarbon, halogenated hydrocarbon, or a mixture of these, wherein the number of hydrogen atom is adjusted to be not less than 20 per 10 halogen atoms, exhibits insufficient water- and hydrogen gas-blocking effect. A carbon coating film of an increased thickness such as to enhance the blocking effect poses another problem of decreased tensile strength of the optical fiber obtained.
In case where a carbon film is industrially formed on the surface of optical fiber, the speed with which a carbon film is formed is not sufficiently fast, and a method for forming carbon film which is industrially more efficient has been desired.
In addition, the present inventors have found that the conventional carbon coat layers are not uniformly applied on fiber, with the result that sufficient water resistance and tensile strength cannot be attained, and that a uniformly applied carbon coat layer can improve these properties.
The conventional reaction apparatuses for carbon coating have a construction wherein carbon coating chamber is not opened. Thus, if a carbon film is formed on the surface of an optical fiber with the use of this reaction apparatus, tile soot component of a thermally decomposed material gas accumulates at the neighbourhood of an exhaust slot of a carbon coating chamber and clogs the slot, whereby a smooth flow of the material gas is prevented, and uniform carbon coating on the optical fiber cannot be achieved. As a result, this carbon coating apparatus permits only 5 to 10 minutes' continuous operation.